期刊
ASTRONOMY & ASTROPHYSICS
卷 435, 期 3, 页码 885-899出版社
EDP SCIENCES S A
DOI: 10.1051/0004-6361:20041170
关键词
ISM : clouds; ISM : molecules; ISM : individual object : Horsehead nebula; radio lines : ISM
We present maps at high spatial and spectral resolution in emission lines of CCH, c-C3H2, C4H, (CO)-C-12 and (CO)-O-18 of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H-2 ro-vibrational lines and in the PAH mid-infrared bands. (CO)-O-18 peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H-2 peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.
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